1. Field of the Invention
This invention relates to the field of digitally-switched impedances.
2. Description of the Related Art
One method of providing a desired impedance is to connect a selectable number of fixed impedances in series. A switching network connects the impedances together in response to a digital input signal. Such a “digitally-switched impedance” may be used, for example, as a potentiometer, rheostat, variable resistor, or a digital-to-analog converter (DAC).
A “resistive” DAC can be provided by connecting the ends of a series-connected impedance string between high and low reference voltages, with the low reference generally being set at ground, and selectively tapping the string to provide a desired analog output voltage. These types of converters are used most commonly as building blocks in MOS analog-to-digital conversion systems, where they function as the DAC subsection of a successive-approximation-type analog-to-digital converter. For an N-bit resistive DAC, the impedance string consists of 2N identical resistors connected in series, and is used as a potentiometer in which the voltage levels between successive resistors are sampled by means of binary switches. Replacing mechanical potentiometers and rheostats is an important and potentially very high volume application for these devices.
FIG. 1 is a schematic diagram of an N-bit DAC that operates on the voltage-scaling principle. A resistor string consisting of resistors R1, R2, R3, . . . , R2N−1, R2N is connected between a high reference voltage (VREF+) node 2 and a low reference voltage (VREF−) node 4, which are typically 5 volts and ground potential, respectively. The voltage drop across each resistor is equal to one least significant bit (LSB) of output voltage change. The output is selected by a switch network, illustrated as switches S1, S2, S3, . . . , S2N. Each switch taps a different point in the resistor string, so that closing a particular switch while leaving the other switches open places a unique analog voltage on a common output line 6 to which each of the switches is connected. A decoder (not shown) receives a digital input signal, and in response, controls the operation of the switches so that the switch whose voltage corresponds to the magnitude of the digital input signal is closed. The signal on analog output line 6 is sensed by a high-impedance buffer amplifier or voltage follower A1, the output of which is connected to an output terminal 8 that provides the final output analog voltage.
A principal drawback of this type of circuit for high-bit-count D/A converters is the very large number of components required: 2N resistors, 2N switches and 2N logic drive lines. For example, in a 12-bit implementation, this approach would use 4,096 resistors, 4,096 switches and 4,096 logic drive lines. It would be highly desirable to significantly reduce this large number of elements for purposes of area savings, higher manufacturing yields and lower costs.
Resistive DACs are presently available which greatly reduce the number of required resistors and switches by using one resistor string consisting of 2N/2 resistors for the digital input signal's most significant bits (MSBs), and a separate resistor string also consisting of 2N/2 resistors for the LSBs. Each resistor in the LSB string has a resistance value equal to ½N/2 the resistance of each MSB resistor. The opposite ends of the LSB string are connected across one of the MSB resistors. By varying the MSB resistor selected for the LSB string connection and taking an output from the LSB string, outputs in one LSB increments can be obtained over the full range of one to 2N−1 LSBs. Two such circuits are the AD569 and AD7846 DACs from Analog Devices, Inc. However, to preserve the constant resistance characteristic of the MSB string, active amplifier buffer circuits must be used to interface between the MSB string and its connection to the LSB string. This unfortunately makes the device unusable for potentiometer and rheostat purposes.
Another reduced parts count resistor-switch configuration for a digital potentiometer is disclosed in U.S. Pat. No. 5,495,245 by James J. Ashe. Referring to FIG. 2 of the Ashe patent, the digital potentiometer uses two outer strings 10 and 12 to provide a decremented voltage pattern that supplies an analog signal corresponding to the MSBs of the digital input signal, while an inner string 14 provides an analog signal corresponding to the LSBs; alternately, the outer strings can provide the LSBs and the inner string the MSBs. The two outer strings 10 and 12 are identical, with the high voltage end of the first outer string connected to a high reference voltage VREF+, and the low voltage end of the second outer string 12 connected to a low reference voltage VREF−. The opposite ends of the inner string 14 are connected to the first and second outer strings through respective outer switch networks that are operated by a decoder (not shown); the decoder in effect causes the opposite ends of the inner string to “slide” along the two outer strings. This “sliding” keeps a constant number of outer string resistors in the circuit, regardless of where the outer strings are tapped. No active elements are required to buffer the inner string from the outer string, which allows the circuit to be used as a potentiometer or rheostat. The output voltage is obtained by tapping a desired location in the inner string 14. In the Ashe invention, each MSB resistor string includes 2N/2−1 resistors and 2N/2 switches, and each LSB string includes 2N/2 resistors and 2N/2 switches. The Ashe digital potentiometer results in a significant reduction in the number of both resistors and switches, compared to the potentiometer circuit illustrated in FIG. 1.
However, the digital potentiometer disclosed in Ashe has inherent non-linearity due to resistor, interconnect and switch resistance mismatches, and may also exhibit long switching settling times caused by large internal capacitances that arise from the parallel connected switches located on the output taps of the MSB resistor strings. Furthermore, though resistor and switch counts are reduced, the disclosed potentiometer still requires a substantial number of both to realize a high-resolution DAC.
U.S. Pat. Nos. 6,201,491 and 6,384,762 to Brunolli et al., and U.S. Pat. No. 6,414,616 to Dempsey, employ a two-stage digitally-switched potentiometer implementation similar to that shown in Ashe. However, as with Ashe, achieving a high resolution still requires a substantial number of resistors and switches, and a correspondingly large die area for their integration.